I definitely recommend seeing the Ferns Field, if you do get a chance. Ken or Addy may be happy to do a tour in return for a bit of help in the gardens.
Other places in the South West. Not
permaculture as such, but really interesting is the Eden project - a botanic garden in a former china clay quarry - like a huge crater garden :). I really likes the lost gardens of Heligan when I first went, but I think it has tamed down a bit over the years.
You could try :
https://www.permaculture.org.uk/land/visiting-and-volunteering as well
and for Scotland (if you make it up North):
https://www.permaculture.org.uk/scotland-centres
These threads may also help:
https://permies.com/t/168981/Public-forest-gardens
from the similar threads:
https://permies.com/t/148379/Permaculture-ish-projects-visit-UK